Apparatus for molding plastic articles

ABSTRACT

A apparatus are disclosed for molding plastic articles, wherein the method includes the steps of filling a mold cavity at a first station with heated flowable plastic material and then displacing the mold to a second station where the plastic material is cooled under pressure. The apparatus includes a rotatable support for displacing the mold between the first and second stations, a plastic supply means at the first station to fill the molds, and a displaceable rod at the second station for insertion into the mold to apply pressure to the plastic material during solidification cooling.

This is a division of application Ser. No. 826,300, filed Aug. 22, 1977,now U.S. Pat. No. 4,150,088, issued Apr. 17, 1979.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method and apparatus for moldingthermoplastic articles, and more particularly to a method and apparatusfor filling a mold cavity at one station and then transferring the moldto a second station where pressure is applied to the thermoplasticmaterial for curing.

2. The Prior Art

Various molding assemblies and methods have been disclosed and utilizedin the prior art to form thermoplastic articles by injection molding.Typically, the methods include injecting thermoplastic material atrelatively high pressure through a rather small sprue opening into amold cavity and then thermally conditioning the material in the mold ata fixed position adjacent the injection assembly. Certain patents, suchas U.S. Pat. No. 3,270,383, have disclosed the use of relatively largesprue openings in the injection process; while a larger group of patentshave generally disclosed the concept of transferring thermoplasticmaterial from an accumulator at relatively low injection pressures.Representative of this last group of patents are U.S. Pat. Nos.3,231,656, 3,453,353, 3,280,236, 3,433,862, 3,947,203, 3,375,553,3,196,198, and 3,092,440. In a related area, Patents such as U.S. Pat.Nos. 3,670,066 and 3,270,383 have disclosed the concept of applyingpressure to thermoplastic material within a mold cavity, but only forthe limited purpose of accounting for shrinkage.

This state of the art has resulted in several previously unsolvedproblems, as follows:

"Flashing" of the plastic material generally occurs between the mold andthe accumulator, even in those methods involving low injection pressuresbecause a high packing pressure is applied to the material in the moldbefore the material cools from a heated flowable condition.

Heat is transferred between the cooled mold and the adjacent heatedaccumulator, resulting in (a) cold slugs in the accumulator and (b)increased cycle time to properly cool the material in the mold.

Plastic material has been subject to degradation due to high injectionpressures, high packing pressures, and unnecessarily prolonged cycletimes.

Additionally, large hydraulic clamping equipment has been required tomaintain the molds closed during both the injection cycle and thecooling cycle. As is common knowledge, such equipment is not onlyexpensive to purchase and operate, but also requires frequentmaintenance.

SUMMARY OF THE INVENTION

The present invention overcomes those prior art problems anddisadvantages in a method and apparatus, wherein a mold cavity is filledat one station with plastic material at low pressure and is thentransferred to a second station to cool the material under pressure.

More specifically, the method includes filling an internal mold cavityat a first station with heated, flowable plastic material. Then, themold is moved from the first station to a second station in alignmentwith a displaceable rod, which is inserted into an opening in the moldto apply pressure to the plastic material while the plastic is cooled toa self-sustaining state. Thereafter, the plastic material is removedfrom the mold cavity as a molded article, and the mold is moved back, tothe first station to receive additional plastic material.

The disclosed apparatus includes a rotatable support member having aplurality of molds mounted on its periphery in circumferentially spacedrelationship. A first station adjacent the periphery of the supportincludes a supply means for filling the molds with plastic material in aheated flowable condition. A second station adjacent the circumferentialperiphery of the support but spaced from the first station includes anaxially reciprocal rod which may be advanced into a sprue opening in themolds by a fluid power means in order to apply pressure to the plasticmaterial during cooling.

In the preferred embodiment, the molds include a female mold componentand a vertical core pin which cooperate to define a tubular, verticalcavity. Additionally, a sprue opening is provided at the top of eachmold to accommodate the injection of thermomplastic material into themold at the first station and to closely receive the rod at the secondstation.

Accordingly, the present invention embodies a significant advance overthe prior art for at least the following reasons:

(1) A significant reduction in flash may be achieved because the plasticmaterial is injected into the mold cavity at a relatively low pressureand because the pressure imposed upon the plastic material by the rod atthe second station may initially be nominal;

(2) Improved cycle times may be achieved due to the elimination of heattransfer between the accumulator and mold and due to the efficient useof the plasticizer and accumulator;

(3) The pressure imposed on the material during cooling may be carefullyregulated by the physically separated curing station in order tominimize material degradation in materials such as polyethyleneterephthalate, which is susceptible to crystallization and to thegeneration of aldehydes;

(4) The cost of the molding equipment may be reduced because largeclamping equipment may be eliminated and because the mold can be mademuch lighter; and

(5) Shear stresses may be minimized during the injection cycle due tothe use of an enlarged sprue opening.

These and other meritorious features and advantages will be more fullyappreciated from the following detailed description and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1--3 schematically illustrate the assembly proposed by thisinvention at various phases of the molding cycle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring more particularly to the drawings, the proposed apparatusgenerally includes a rotatable turret 10 to which two separate moldingassemblies 40 and 50 are secured for displacement between an injectionstation 60 and a curing station 70.

The rotatable turret or support column 10 is rotationally mounted abouta vertical cylindrical beam or rod 12 which is secured to base 13. Apinion 14 is integral with the bottom of turret 10 and meshes with arack 16 which is displaced by a piston 18 of a hydraulic cylinderarrangement 20.

The mold assemblies 40 and 50 are essentially identical and thereforethe description of these assemblies will be devoted primarily to mold40, with like reference numerals in mold 50 indicating identical parts.

Mold assembly 40 includes a female mold member 41 which is suitablysecured to a peripheral surface on the turret 10. The female moldincludes an essentially cylindrical, vertical mold cavity 42 whichblends into a sprue opening 43 at the top of the mold. A verticallymovable male mold member 44 includes an elongated vertical core pin 45which cooperates with cavity 42 to define an essentially tubular,vertical mold cavity. The male mold member is vertically displaced by anelongated piston 46 of a hydraulic cylinder 47.

As shown, the female mold member includes internal cooling ducts 48through which fluid is circulated to cool plastic material within themold cavity. Likewise, the elongated core pin 45 may include an internalcooling system (not shown) as is conventional in the art. For thermalsegregation purposes, a layer of insulation 49 is secured to the top ofthe female mold 41 for purposes which will become apparent from thefollowing disclosure.

The first station 60 includes a fixed, heated accumulator 61 whichincludes a main accumulator chamber 62 and a flow passageway 63, withwhich sprue opening 43 is selectively aligned. A suitable plasticizer 64including a rotatable screw 65 selectively delivers plastic material ina heated, flowable condition to the accumulator 62 by way of passageway66. An elongated, cylindrical ram closes off the opposed end ofaccumulator 62 and is selectively displaced by a piston 68 of ahydraulic assembly 69.

Station 70 includes a fixed hydraulic cylinder 71 and an associatedvertically reciprocal pressurizing rod 72 which may be selectivelyadvanced into the molding assemblies positioned thereunder to pack theplastic material during solidification cooling. The rod 72 is preferablyon the order of about a half inch in diameter and conforms closely tothe diameter of the mold sprue openings.

In operation, a molding cycle is initiated by retracting ram 67 fromwithin accumulator 62 to the position shown in FIG. 1. As a result,heated, flowable plastic material is delivered into the accumulatorchamber 62 from the plasticizer 64. As shown in FIG. 1, this cycle hasjust been initiated whereas in FIG. 2 the cycle has progressed to thepoint where the accumulator chamber 62 is substantially filled withplastic material. At about the time in the cycle represented by FIG. 2,or very shortly thereafter, hydraulic cylinder 69 is actuated to advancepiston 68 and ram 67 into the accumulator chamber 62 to the positionshown in FIG. 3 for the purpose of injecting plastic material into themold cavity by way of sprue opening 43. As will be appreciated, optionalvalving (not shown) may be provided in the accumulator 61 in the regionof flow passageway 63 to regulate the flow of plastic material fromchamber 62 to the mold assembly.

In accordance with the present invention, the plastic material isinjected into the accumulator chamber 62 from the plasticizer 64 andthen into the mold cavity from the accumulator 62 at relatively lowinjection pressures. As a result, the injection molding temperatures maybe maintained at a lower level than in prior methods, providing theadvantages previously enumerated. It will be understood, of course, thatpressures on the order of up to about 5,000 psi may be developed at thepoint in the cycle represented by FIG. 3 simply as a result of the sizeof the plastic shot being slightly greater than the mold cavity size.Even if this occurs, it is considered to be an incidental effect of theinjection cycle and does not appear to diminish the overall advantagesof the invention.

Also in accordance with the invention, the flow passageway 63 and thesprue opening 43 are both relatively large, such as on the order ofabout half an inch. As a result, shear stresses in the plastic materialare significantly reduced.

After the point in the cycle represented by FIG. 3, hydraulic cylinder20 is actuated to advance piston 18 and rack 16 for the purpose ofrotating the turret 10 and displacing mold assembly 40 to a positionbeneath pressurizing rod 72 as represented by the position of moldassembly 50 in FIG. 1. At about the same time, ram 67 is retracted bypiston 68 back to the position shown in FIG. 1. Withdrawal of the ram 67tends to pull plastic material within passageway 63 back intoaccumulator 62 and also accommodates the flow of additional plasticmaterial from plasticizer 64.

As is conventional in the art, cooling fluid is continuously circulatedthrough the internal cooling ducts of the mold assemblies. Thus, thecooling cycle starts as soon as plastic material is forced from theaccumulator into the mold assembly. However, the plastic material isstill in a heated, uncured condition as the molds are rotated from thefirst station 60 to the second station 70.

Referring now more particularly to mold assembly 50 to explain theoperation at station 70, rotation of the turret 10 positions sprueopening 53 in vertical alignment with the pressurizing rod 72.Thereafter, hydraulic cylinder 71 is actuated to advance pressurizingrod 72 into the sprue opening to apply pressure to the plastic materialwithin the mold cavity during the solidification cooling process.

In accordance with the invention, the pressure applied by the rod 72 maybe regulated as desired. For example, either a constant low or highpressure may be applied to the material to compensate for shrinkage.Alternatively, the pressure applied by rod 72 may be varied as desiredfor several different purposes. For example, an initial low pressure maybe applied to minimize flashing, with a higher pressure being appliedafter the material has cooled from a flowable condition. As anotherexample, the pressure may be varied to control properties inthermoplastic materials, such as crystallization in polyethyleneterephthalate.

After the plastic material with the mold cavity has cooled to a selfsustaining condition, piston rod 56 is retracted by the hydrauliccylinder 57 to withdraw core pin 55 from the mold cavity 52 as shown inFIG. 3. Thereafter, the thermoplastic material may be stripped from thecore pin in a conventional manner and then blown into a container in ablow molding operation. Next, rod 56 is extended to reposition core pin55 within the mold cavity, and the turret 10 is rotated to repeat theprocess.

Having fully and completely described our invention, we now claim:
 1. Anapparatus for forming a plastic article, comprising:a movable moldhaving an internal mold cavity and a sprue opening; supply means at afirst station for filling the internal mold cavity and sprue openingwith heated, flowable plastic material; low pressure filling means atthe first station to fill the mold at a pressure of less than about 5000psi; displacement means for moving the mold immediately after fillingfrom the first station to a second station where the sprue opening is inalignment with an axially reciprocal pressurizing member; and fluidpower means for (a) axially displacing the pressurizing member into thesprue opening to apply pressure to the plastic material within theinternal mold cavity and (b) maintaining pressure greater than the moldfilling pressure on the member while the plastic material within theinternal mold cavity is thermally conditioned to a self-sustaining stateto provide a hollow plastic article.
 2. The apparatus as defined inclaim 1, including a rotatable turret interconnected to both saiddisplacement means and the mold, said turret being mounted for rotationabout an axis and the mold being movable with the turret along anarcuate path between said first and second stations.
 3. A mold assembly,comprising:a support member mounted for rotation about an axis; aplurality of molds mounted on the periphery of the support member incircumferentially spaced relationship, with each of the molds includinga sprue opening and cooling means; a first station adjacent thecircumferential periphery of the support member and including lowpressure filling means for successively filling the molds with plasticsmaterial polyethylene terephthalate in heated, flowable condition at apressure of less than about 5000 psi; means for transferring the moldsimmediately after filling to a second station; the second station beingadjacent the circumferential periphery of the support member andincluding an axially reciprocal rod and a fluid power means foradvancing the rod into the mold sprues to apply pressure to the plasticmaterial within the molds at a pressure greater than the fillingpressure while said cooling means cools the plastic material to aself-sustaining condition to obtain an hollow plastic article.
 4. Theapparatus as defined in claim 3, wherein each of the molds includes afemale mold and a vertical core pin cooperatively defining anessentially tubular vertical cavity; and the sprues being at the top ofeach mold cavity.